This invention relates in general to electrophotography and in particular, to electrophotoconductive imaging members having crack-resistant multiple layers.
In electrophotography utilizing a liquid development system, an electrophotographic plate, drum, belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. The electrostatic latent image is developed to form a visible image by applying an electrically charged liquid toner. The resulting visible image is transferred from the imaging member directly or indirectly to a support such as paper. The liquid toner can contain various types of colorant and dye attached to a resin dispersed in an insulating liquid carrier.
Higher speed electrophotographic copiers, duplicators and printers place stringent requirements on photoreceptors. The numerous layers found in many modern photoconductive imaging member belts must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
Long service life is required in an imaging member. Compact imaging machines employ small diameter photoreceptor belt system support rollers. Small diameter support rollers are desirable for simple, reliable copy paper stripping systems that utilize beam strength of copy paper to automatically remove copy paper sheets from the surface of a photoreceptor belt after toner image transfer. Small diameter rollers, e.g. less than about 0.75 inch (19 mm) diameter, raise the threshold of mechanical performance criteria of photoreceptors to a high level. Spontaneous photoreceptor belt material failure becomes a frequent event.
One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer a charge generating layer and a charge transport layer. The charge transport layer may comprise an electrically active small molecule dispersed or dissolved in an electrically inactive polymeric film forming binder. The expression "electrically active" means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer to discharge a surface charge on the active layer.
The multilayered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, an adhesive layer and an overcoating layer.
Photoreceptors may suffer from cracking, crazing, crystallization of active compounds, phase separation of activating compounds and extraction of activating compounds by organic carrier fluid such as isoparaffinic hydrocarbons, e.g. Isopar.RTM., that are commonly employed in an electrophotographic imaging system using liquid developer inks. The effects of carrier fluid can markedly degrade mechanical integrity and electrical properties of the photoreceptor. The organic carrier fluid tends to leach out activating small molecules typically used in the charge transport layers. The leaching process results in crystallization of the activating small molecules, such as arylamine compounds, onto the photoreceptor surface with subsequent migration of arylamines into the liquid developer ink. In addition, the carrier fluid can induce the formation of cracks and crazes in photoreceptor surface. These effects lead to copy defects and shortened photoreceptor life. Degradation of the photoreceptor manifests itself as increased printing defects prior to complete physical photoreceptor failure. Leaching out of the activating small molecule can increase susceptibility of the transport layer to solvent/stress induced cracking when the belt is parked over belt support rollers or when the belt is subjected to dynamic fatigue flexing during imaging process. Cracks developing in charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality. Furthermore, cracks in the photoreceptor pick up toner particles that cannot be removed in a cleaning step. The particles can subsequently be transferred and deposited onto a receiving member to cause increased background defects on prints. Crack areas are subject to delamination when contacted with blade cleaning devices thus limiting electrophotographic product design. Some carrier fluids promote phase separation of the activating small molecules, particularly when high concentrations of the arylamine compounds are present in a transport layer binder. Phase separation can adversely alter electrical and mechanical properties of a photoreceptor.
Flexing is normally not encountered with rigid, multilayered photoreceptor drum configurations that utilize charge transport layers containing activating small molecules dispersed or dissolved in a polymeric film forming binder. Nonetheless, electrical degradation can be encountered during development with liquid developers. Degradation of these photoreceptors by liquid developers can occur in less than eight hours of use to render a photoreceptor unsuitable for even low quality xerographic imaging purposes.
Photoreceptors having charge transport layers containing charge transporting arylamine polymers are described in the patent literature. These polymers include the products of a reaction involving a dihydroxy arylamine reactant and are described for example in U.S. Pat. Nos. 4,806,443, 4,806,444, 4,801,517, 5,030,532 and 4,818,650, the entire disclosures of these patents being incorporated herein by reference. Although arylamine transporting polymers overcome many of the problem of binder/small molecule systems, they may not meet all of the functional requirements of some sophisticated electrophotographic systems. Specifically arylamine transporting polymers exposed to hydrocarbon ink vehicles can exhibit dynamic fatigue cracking when cycled around narrow diameter rollers. In addition, thermal contraction mismatch can occur during photoreceptor fabrication resulting in a photoreceptor possessing a curl. When forced flat either mechanically or by the application of an opposite curling back coating, stress is applied to the transport layer. This can aggrevate the cracking problem.
In summary, organic based photoreceptors, even those using polymeric arylamine transport materials, can suffer from dynamic fatigue cracking, especially when the photoreceptor is in the form of a belt cycling over small diameter rollers while in contact with an hydrocarbon ink vehicle.